The invention relates to a projection-lens system or lens for projecting a magnified image of a scene reproduced by means of a reproduction element onto a projection screen. The lens system comprises, in this order and viewed from the image side, a first group having one aspheric surface, a positive second or main group having two spherical convex outer surfaces, and a third group comprising a planoconvex lens whose concave surface which faces the image side is aspheric, the elements of the first group and of the third group being made of a transparent plastics material. The invention also relates to a colour-television projection system provided with at least one projection-lens system of this kind.
Such a projection-lens system or lens is disclosed in the U.S. Pat. No. 4,348,081, which system is used for projecting a scene in one colour onto a projection screen, where superposition of three monochrome images results in a colour picture. The third lens group from the image side is a field-curvature correction lens element, also referred to as "Field flattener", which provides compensation for the Petzval curvature of the two other lens groups. The lens shown in FIG. 1 of U.S. Pat. No. 4,348,081 has a main group comprising one biconvex lens. If this lens element is made of glass, which is a stable material, the focal length, which largely determines the focal length of the entire system, is substantially temperature-independent. The two other groups are made of a transparent plastic which is lighter in weight than glass, so that the weight of the entire projection-lens system remains within acceptable limits despite the fact that the lens elements have comparatively large diameters, for example of the order of 100 mm. Since a part of the power of the entire lens system is provided by the first group and the third group, which are made of a plastic whose refractive index is temperature dependent, the focal length of the entire system is still temperature dependent.
Moreover, the projection-lens system of Pat. No. 4,348,081 provides the desired imaging only for a small range of wavelengths around the central wavelength of the radiation which passes through the system; for radiation in a wider range of wavelengths the picture will not be in focus owing to the variation in focusing as a function of the wavelength. In view of the comparatively large bandwidth of the radiation emitted by the reproduction tubes, in particular the reproduction tube with the green phosphor, it is desirable that in practice an achromatic main lens is used.